Bone islands incidentally detected on computed tomography: Frequency of enostosis and differentiation from untreated osteoblastic metastases based on CT attenuation value

Objective: The frequency of enostosis incidentally found on CT and CT attenuation value to distinguish them from untreated osteoblastic metastases (UOM). Methods: Enostosis group: 46 polytrauma patients underwent thoracoabdominal CT. Inclusion criteria: age range 14-35 years. Exclusion criteria: cancer, previous fractures. UOM group: 20 patients with radiological diagnosis of UOM. Analyzed data: number, size, location and density of enostoses and metastases. The density was measured with the broadest possible region of interest at the center of the lesion by two radiologists independently. Receiver operatingcharacteristic analysis to determine the sensitivity and specificity, area under the curve 95% confidence intervals and cutoff values of CT density to differentiate metastases from enostoses. Results: Patients were 28 ± 7 years old (72% males). 41 (89%) patients had 124 enostoses (2-15 mm) with an average density of 1007 ± 122 Hounsfiled unit (HU, observer1) and 1052 ± 107 (observer2). The most common sites of occurrence were the proximal femur (34%), the pelvis (22%), the acetabulum (20%), the proximal humerus (11%), the vertebrae (11%) and the rib (2%). 13 patients had 1 bone island, 8 patients had 2, 9 cases had 3 and 11 cases had more than 3 enostoses. Overall, 114 UOM were evaluated, their average density was 728 ± 163 HU (observer1) and 712 ± 178 HU (observer2). The area under the curve value of mean density to distinguish enostoses from UOM was 0,982. Using a cut-off of 881 HU for mean density, sensitivity was 98% and specificity 95%. Conclusion: The frequency of enostosis in this study is 89%. The average density identified can help to distinguish enostoses from UOM. Advances in knowledge: We report the exact frequency of enostosis.